1. Field of the Invention
The invention relates to a discharge lamp such as a metal halide lamp, a halogen electric bulb, high-voltage discharge lamp or other similar lamp and, more particularly, to a discharge lamp configured by sealing metal foils within sealing portions of a bulb.
2. Description of the Related Art
Metal halide lamps are conventionally configured, for example, as shown in FIG. 13. That is, in FIG. 13, a metal halide lamp 1 comprises a hollow glass tube bulb 2, a pair of discharge electrodes 3 and 4 arranged within the glass tube bulb 2 and metal foils 7 and 8 connecting the discharge electrodes 3 and 4 to externally extending lead wires 5 and 6.
The glass tube bulb 2, made, for example, of quartz glass, is formed roughly in the form of a hollow sphere in the case of the illustration. At the same time, the glass tube bulb 2 is provided with sealing portions 2a and 2b each at an axial end, and mercury/metal halide, etc. is enclosed in the sealing portions 2a and 2b when these portions are sealed.
The discharge electrodes 3 and 4, made, for example, of a metal such as molybdenum, are arranged such that they are opposite to each other with a given spacing roughly at the center of the glass tube bulb 2.
The lead wires 5 and 6, similarly made of a metal such as molybdenum, are intended to supply power to the discharge electrodes 3 and 4 and are electrically connected to the discharge electrodes 3 and 4 via the metal foils 7 and 8. The metal foils 7 and 8, made, for example, of molybdenum foils, are sealed within the sealing portions 2a and 2b at respective ends of the glass tube bulb 2.
More specifically, with the discharge electrode 3 and 4 and the lead wires 5 and 6 connected respectively, for example, by welding, the metal foils 7 and 8 are inserted into the sealing portions 2a and 2b in an open state at both ends of the glass tube bulb 2, and then the sealing portions 2a and 2b are respectively softened by heating and crushed flatly so as to hold the metal foils 7 and 8, thus sealing the metal foils 7 and 8. This ensures hermetic sealing of the metal foils 7 and 8—portions that function as power supply portions for connecting the discharge electrodes 3 and 4 and the lead wires 5 and 6—within the sealing portions 2a and 2b of the glass tube bulb 2, thus maintaining the internal space of the glass tube bulb 2 hermetic.
Here, while the metal foils 7 and 8 have an approximately 10-fold higher thermal expansion ratio than quartz glass, the metal foils 7 and 8 are formed such that they are extremely thin, thus allowing hermetic sealing of the inside of the glass tube bulb 2.
Incidentally, in the metal halide lamp 1 thus configured, since there is a considerable difference in thermal expansion ratio between a metal such as molybdenum (which make up the metal foils 7 and 8) and quartz glass (which make up the glass tube bulb 2 as described earlier), the amount of expansion and compression due to temperature change is likely largest in the axial direction of the metal foils 7 and 8. This leads to stress concentration in longer sides extending along the axial direction of the metal foils 7 and 8.
As a result, there are times when cracks, originating from the longer sides of the metal foils 7 and 8, occur in the sealing portions 2a and 2b of the glass tube bulb 2. Cracks can prominently occur, particularly if the metal foils 7 and 8 are subjected to rough surface finish.
As a countermeasure therefore, there is disclosed in Japanese Patent Application Laid-Open Publication No. 1999-7918, a molybdenum foil glass sealing portion 9 in which axial end edges of a molybdenum foil are formed in a wedge shape as viewed in cross-section.
Based on this configuration, deformations such as burrs arising out of cutting of molybdenum foils are resolved by forming the cut edge in a wedge shape, thus suppressing the occurence of cracks, originating from longer sides of molybdenum foils, within the sealing portions of the glass tube bulb.
In the molybdenum foil glass sealing portions according to Japanese Patent Application Laid-Open Publication No. 1999-7918, however, it has been discovered that if the molybdenum foils are rough finished on their surface, it is difficult to reliably suppress the occurence of cracks originating from longer sides of the molybdenum foils within the sealing portions of the glass tube bulb. Thus, such a configuration typically results in a crack, for example, about 1.5 mm in size occurring immediately after sealing of the molybdenum foils.
In addition, repetitive flashing of the metal halide lamp 1 gives rise to cracks both in longer sides of the metal foils 7 and 8 and in the entire surrounding area, and expands already existing cracks. This is presumably caused by stress concentration not only in longer sides but also in shorter sides and corners of the metal foils 7 and 8 due to the difference in thermal expansion ratio resulting from temperature variations. In the event of expansion of such cracks, cracks reach the outer surface under certain circumstances, possibly causing mercury/metal halide, etc. that is enclosed within the glass tube bulb to leak externally.
This kind of problem is not limited to metal halide lamps and occurs in other lamps such as those including tungsten-halogen electric bulbs and high-pressure discharge lamps in which metal foils making up power supply portions are sealed within the sealing portions of the glass tube bulb.